An aerosol, as used herein, is a relatively stable suspension of fine solid or liquid particles in a gas, especially air. The atmosphere contains a vast array of trace particulates. These particles may be biological, chemical, or radiological in origin, and may result from terrestrial processes such as weather, fires, volcanoes and earth quakes, animal and human activities, and other sources. At any given time, an air sample may contain minerals, dust, pollen, mold, live and dead microorganisms and their products, trace chemical vapors, and so forth, and the mix is constantly changing.
An increasing number of systems are used to detect and analyze aerosols, ranging from systems that measure and monitor the amount of pollen, dust, mold and pollutants in the air, to systems that detect minute amounts of highly toxic biological, chemical and radiological warfare agents. Such systems may be designed for open-air environment point detection applications and/or indoor point detection applications. Both high level and low level monitoring systems are in use. For example, high level monitoring systems may be used in industrial applications where very high concentrations of aerosols are found, such as in sawmills, foundries, livestock facilities, and the like. Lower level monitoring systems are needed in industrial and healthcare applications such as semiconductor fabrication “clean rooms,” and hospital operating rooms. Still other highly sensitive systems are designed for detection and monitoring of airborne chemical and biological warfare agents.
An aerosol collector is generally used by detection and monitoring systems to extract and concentrate aerosol samples from the surrounding environment which are then analyzed for target aerosols. While a number of different sampling methods are employed depending on the application, aerosol collectors typically rely on a consumable collection media that requires servicing from time to time. Thus, the collectors are configured to operate only when there is a reason to suspect that there may be aerosol particles of interest in the surrounding air. The task of deciding when to activate the collector is performed by a “trigger” or “cue” component. In certain sophisticated aerosol detection systems, for example, the trigger component continually samples the air and interrogates aerosol particles for fluorescence signatures that characterize particles of interest. When it detects a characteristic aerosol particle in sufficient concentration it “triggers” the collector to collect a sample for detailed analysis. Unfortunately, the presence of certain particulates in sufficient numbers may retard or inhibit the trigger's ability to accurately detect the presence or absence of particulates of interest resulting in false triggering or other malfunctions. For example, diesel exhaust, road dust, burning vegetation, ethylene glycol vapor and fog oil are all known to cause triggering errors in aerosol detection systems. Such triggering errors can result in premature system failure, false positive and false negative detection errors and increased servicing costs.
A variety of hardware and software active and passive filter elements have been devised to compensate for the presence of interferents in order to reduce false triggering events. Although such filtering elements add to the cost and complexity of a detection system, the results have been less than satisfactory and interferent rejection continues to be a significant problem for aerosol detection systems.
Embodiments according to the present invention employ a different approach from such filtering and compensation systems and substantially reduce the likelihood of triggering errors by providing an elegant and highly effective flow modulator at the input to a triggering/cueing component of an aerosol detection system.